There are many different types of noise that affect digital subscriber line (DSL) technology, such as impulse noise, radio frequency interference (RFI) and crosstalk. Also, stationary noise typically causes reduced data transfer rates due to lowered signal to noise ratio (SNR) while non-stationary noise in form of e.g. impulse noise, fading RFI and fluctuating crosstalk typically cause stability problems for DSL.
While impulse noise and RFI are only present in certain environments, crosstalk is always present when there are other active communication lines (disturbers) in a telecommunication cable (cable) or cable binder (binder). As is known within the art, a binder is a subgroup of the cable. The cable consists of two or more communication lines arranged adjacent to each other, where one communication line (also referred to as “line” or “pair”) typically comprises a pair of twisted wires (pair). Cables with many pairs are usually subdivided into binders where pairs within a binder typically experience stronger crosstalk between each other compared with pairs in different binders.
A typical noise environment seen by a DSL transceiver is thus dominated by crosstalk. Crosstalk depends on many factors such as the quality of the communication line, the number of disturbers and their transmit power spectral density (PSD). The crosstalk typically comprises far end crosstalk (FEXT) and near end crosstalk (NEXT), where FEXT in a line is interference, from one or more adjacent lines, as measured at the end of the line farthest from the adjacent lines' transmitters, while NEXT is interference from one or more adjacent lines, as measured at the end of the line nearest to the adjacent lines' transmitters.
Noise in general and crosstalk in particular is a major cause of capacity limitation for DSL technology. Since DSL is based on differential mode transmission, the level of external noise coupled into the communication line is dependent on the so called “pair symmetry”, which is a measure of the similarity in coupling of the external noise into each wire in a pair (communication line). If the coupling is identical, both wires will contain identical noise signals and thus the noise will be completely cancelled by the differentially coupled receiver connected to the line. Such perfect pair symmetry will not be the case in conventional communication lines.
Hence, knowing the pair symmetry is relevant because it affects the noise in the communication line. Also, pair symmetry can be used as an indicator for a group of faults that affect the performance of the line, and may indicate whether the line needs to be repaired. A prior art method for detecting problems with pair symmetry includes comparing a measured noise PSD with a constant threshold value, e.g. by investigating if the average noise level in a certain frequency band is above a certain threshold.
If pair symmetry in a communication line is poor the so called line balance is frequently also poor. Line balance is often referred to as longitudinal conversion loss (LCL) of a twisted pair (i.e. twisted wire) communication line (P. Golden et al, “Fundamentals of DSL Technology”, Auerbach Publications, USA, 2006).
Prior art in the area of detecting problems with line balance and high noise levels are commonly based on LCL measurement with dedicated metallic line testing (MELT) instruments, or based on using test functionality integrated in POTS (plain old telephone service) linecards. An example of a measurement setup to determine line balance is described in ITU-T (International Telecommunication Union) Recommendation G.996.1, “Test procedures for digital subscriber line (DSL) transceivers”, February 2001.
A problem with prior art in the area of identifying pair symmetry by comparing a measured noise PSD with a constant threshold is that it is rather inaccurate. One line may exhibit a particular noise level due to a fault whereas the same noise level may be found in another line that is functioning normally.
Determining line balance according to prior art techniques has also some disadvantages, e.g. because necessary measurements require galvanic access to the individual wires in the communication line as well as to a ground reference. Traditionally, this has been accomplished by using either dedicated instruments or test functionality built into the POTS linecards. For remote deployment of DSLAMs (e.g. in cabinets) it is relative expensive to manually connect test instruments in order to check if there is a fault on the line. Also, manual testing is labour intensive and takes significant time to complete, which prevents first line support when e.g. talking to a customer. Further, since Voice over IP is increasingly replacing traditional POTS for telephony, testing functionality in POTS linecards will not always be available.